Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and the like, are configured to communicate over wireless networks. Accordingly, each such portable communication device relies on a transmitter and receiver (or transceiver), typically connected to a single or common antenna, for sending and receiving data and control signals over the wireless network. In order to use the common antenna, a duplexer is included to interface between the common antenna and each of the transmitter and receiver, so that the transmitter is able to send signals on a transmit frequency and the receiver is able to receive signals on a different receive frequency. Generally, the duplexer includes two band-pass filters having different passbands for filtering the transmit and receive signals, respectively, thus preventing or reducing interference between the transmit and receive signals.
Various types of wireless network are implemented according to different communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), personal communications services (PCS), digital cellular system (DCS), international mobile telecommunication (IMT), and enhanced data rates for GSM evolution (EDGE). The communication standards identify separate bands for transmitting (uplink) and receiving (downlink) signals. For example, UMTS band 2 (PCS) provides an uplink frequency band of 1850 MHz-1910 MHz and a downlink frequency band of 1930 MHz-1990 MHz; UMTS band 3 (DCS) provides an uplink frequency band of 1710 MHz-1785 MHz and a downlink frequency band of 1805 MHz-1880 MHz; UMTS band 7 (IMT-E) provides an uplink frequency band of 2500 MHz-2570 MHz and a downlink frequency band of 2620 MHz-2690 MHz; and UMTS band 8 (GMS-900) provides an uplink frequency band of 880 MHz-915 MHz and a downlink frequency band of 925 MHz-960 MHz. Accordingly, a duplexer operating in compliance with a UMTS standard would include a transmit filter having a passband within the corresponding uplink frequency band, and a receive filter having a passband within the corresponding downlink frequency band.
Demand for smaller, less expensive and more efficient portable communication devices is significant. Therefore, reducing size and weight of portable communication devices, as well as reducing fabrication costs and increasing product yield, are priorities. For example, there is demand for the band-pass filters of duplexers in portable communication devices to be smaller, to consume less power, to have improved performance characteristics (such as lower insertion loss and higher out-of-band attenuation), and to operate at higher frequencies. Such duplexers may include resonators for filtering the transmit and receive signals, such as a thin film bulk acoustic resonators (FBARs). However, design and fabrication are difficult, e.g., due to passband and stopband requirements of the corresponding receive and transmit band-pass filters, and matching circuit requirements between the band-pass filters and the antenna.